Yeah, that was my original thought, invest at higher rates... i just know that paying down the 4.75% would be a "sure win" when using 3.375% money. Also, by taking on a higher refi than i need, but still retaining both mortgages, i would still be able to depreciate, and would only lose out on the incremental tax deductible mortgage interest difference from the 3.375% loan vs. the 4.75%. I think that's a negligible amount.
There are those people who would say don't borrow against your house to invest. Losing $ on investments is bad, losing someone else's $ could be worse, but losing your house is unthinkable. That being said, i don't think it'd come to that- we have ample $ in the bank to payoff the full loans completely if disaster struck... but would likely mean selling shares at lower prices.